Waveguide modulators structures

ABSTRACT

A Mach-Zehnder waveguide modulator comprising a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a National Phase Patent Application of PCT/GB2016/050570, filed on Mar. 4, 2016, which claims the benefit of U.S. Provisional Patent Application No. 62/128,949, filed on Mar. 5, 2015 and 62/152,696, filed on Apr. 24, 2015. The entire contents of all of the above-identified documents are incorporated herein by reference.

FIELD

One or more aspects of embodiments according to the present invention relate to waveguide modulator structures and methods, and more particularly to Mach Zehnder waveguide modulator structures and methods.

BACKGROUND

Optical modulation on a silicon waveguide platform is well known including waveguide modulators based upon: electro-optic, quantum confined Stark, Franz-Keldysh, and carrier plasma dispersion.

Waveguide modulators can include silicon, III-V, other semiconductors, and polymer waveguides. They are typically symmetrical structures, although doped areas may not be symmetrical (e.g. U.S. Pat. No. 7,085,443, PN phase shift modulator) where the doping of silicon is asymmetric across the waveguide).

For modulators with a silicon/germanium (SiGe) medium, it is known that the Franz-Keldysh (FK) effect with lumped electrodes is used to achieve small size, high speed, low driving power consumption and ease of manufacture at 1550 nm. A silicon/germanium region provides an electro-absorption effect that is advantageous when operation is required in the C- and L-bands of the optical spectrum.

Several possible modulator waveguide structures have been proposed including U.S. Pat. No. 8,346,028 where the active waveguide is made of (typically) SiGe and the SiGe waveguides are doped on their sides to provide effective electric contacts and to generate the necessary electric field in the SiGe waveguide.

However, such a structure suffers from the drawback that the optical mode is forced to be in close proximity to doping on both sides of the waveguide, which causes optical losses.

SUMMARY

According to various embodiments of the present invention, there is provided a Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a junction region and a plurality of electrodes for providing a bias across the junction to enable control of the phase of light travelling through the junction regions via dispersion.

Dispersion may be understood to be the dispersion effect and more particularly an enhanced plasma dispersion effect.

According to a first set of embodiments, there is provided a Mach-Zehnder waveguide modulator comprising a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a PIN junction region, the PIN junction formed from: a first semiconductor region corresponding to either the p-doped region or the n-doped region; a second semiconductor region corresponding to the other of the p-doped or n-doped region; and a central SiGe waveguide region; wherein the first semiconductor region of the left optical waveguide is integral with the first semiconductor region of the right optical waveguide in a region between the left and right arms, forming a common doped region; and a plurality of electrodes configured to apply a forward bias applied across one or both of the PIN junctions to enable control of the phase of light travelling through the respective PIN junction region(s) via dispersion.

According to a second set of embodiments, there is provided a Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a PIN junction region, the PIN junction formed from: a first semiconductor region corresponding to either the p-doped region or the n-doped region; a second semiconductor region corresponding to the other of the p-doped or n-doped region; and a central SiGe waveguide region; wherein the first semiconductor region of the left optical waveguide is electrically isolated from the first semiconductor region of the right optical waveguide by a central isolated region between the left and right arms, forming a common doped region; and a plurality of electrodes configured to apply a forward bias applied across one or both of the PIN junctions to enable control of the phase of light travelling through the respective PIN junction region(s) via dispersion.

In U.S. Pat. No. 8,346,028 and U.S. Pat. No. 8,093,080 are described waveguide structures for modulators and light sensors where both side walls of the waveguide are appropriately doped. The advantage of side wall doping is that the electric field necessary to operate the FK effect is closely contained in the waveguide but the disadvantage is that the doping close to the optical mode absorbs light and causes a great deal of optical loss.

U.S. Pat. No. 8,737,772 discloses a device which is a depletion phase modulator with carrier plasma dispersion effect and in which the p- and n-doped regions are disposed asymmetrically with the one doped area extending to the top to the waveguide and the opposite doped region being in the base. The purpose of this structure is to reduce optical loss by forming a depletion region in the rib waveguide and distancing the electrical contact (metal) from the optical mode.

In the symmetrically side wall doped structures of the prior art, including those of U.S. Pat. No. 8,346,028 and U.S. Pat. No. 8,093,080, the electric field created by applying a voltage across is an essentially a parallel and uniform field. In this invention we disclose a novel structure in which the electric field created across doped regions is non-parallel and non-uniform but the optical mode is effectively unaffected by the asymmetry of doping. Consequently, in the structure of the present invention, the loss is less because there is doping only on one side of the waveguide but the electric field, although not uniform, is effective in producing the required FK effect. Thus a high speed modulator is achieved without the disadvantages of prior designs such as that of U.S. Pat. No. 8,346,028.

The structure can also be used for photo-detectors with germanium replacing SiGe. Comparing to the prior structures such as that of U.S. Pat. No. 8,093,080, the structures of the present invention have higher responsivity due to the lower optical loss.

According to a third set of embodiments, there is provided a Mach-Zehnder waveguide modulator according to claim 1, wherein the junction of each of the left and right optical waveguides comprises a PN junction, the PN junction formed from: a first semiconductor region corresponding to either the p-doped region or the n-doped region; a second semiconductor region corresponding to the other of the p-doped or n-doped region; and a plurality of electrodes configured to apply a reverse bias across one or both of the PN junctions to enable control of the phase of light travelling through the respective PN junction region(s) via dispersion.

Each of the first set and the second set of embodiments may include one of or any combination of the following optional features. Where these features do not relate to the intrinsic region, they are also applicable to the third set of embodiments.

Optionally, the first semiconductor region includes a first lateral portion extending laterally away from the waveguide wall on a first side of the waveguide; the second semiconductor region includes a second lateral portion extending laterally away from the waveguide wall on a second side of the waveguide; and the plurality of electrodes comprises: a first electrode located directly on top of the first lateral portion; and a second electrode located directly on top of the first lateral portion.

Optionally, one of the first or second semiconductor regions includes a vertical doped portion which extends along the side of the waveguide.

Optionally, the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped region.

Optionally, the vertical doped portion extends along the entire side of the waveguide.

Optionally, for a given waveguide arm of the modulator, no more than one side of the waveguide comprises a vertically doped portion.

Optionally, the Mach-Zehnder waveguide modulator comprises: an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region; wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region.

Optionally, the Mach-Zehnder waveguide modulator comprises: intervening lightly n-doped semiconductor region located between the n-doped semiconductor region and the central waveguide region; wherein the intervening lightly n-doped semiconductor region has a lower dopant concentration than the n-doped semiconductor region.

Optionally, the central waveguide is composed of SiGe and the doped semiconductor regions are composed of Si.

Optionally, the central waveguide region, the first semiconductor region, and the second semiconductor region are all composed of SiGe.

Optionally, the central waveguide region, the n-doped semiconductor region, the p-doped semiconductor region, the lightly p-doped semiconductor region, and the lightly n-doped semiconductor region are all composed of SiGe.

Optionally, the plurality of electrodes includes a common electrode located at the common doped region.

Optionally, the plurality of electrodes includes a plurality of lumped electrodes.

Optionally, the Mach-Zehnder waveguide modulator is configured to operate in a push-pull mode.

Optionally, in all embodiments described herein, the SiGe waveguides are formed from intrinsic SiGe (iSiGe).

Optionally, the active region means that waveguides in this region have PIN or PN junctions, and the electrodes can be either lumped electrodes or traveling wave electrodes. The waveguide in the active region is made of SiGe material, outside of this region may be made of SiGe or Si material.

For Mach-Zehnder modulators with PIN junctions, the p region may be SiGe material or Si material doped with boron, and the n region may be SiGe or Si material doped with phosphorus or arsenic. During operation, the PIN junction may be forward biased so that carriers are injected into the intrinsic region, resulting in the desired dispersion.

The intrinsic region is not intentionally doped, but may exhibit some background doping.

For Mach-Zehnder modulators with PN junctions, the p region has SiGe material doped with boron, and the n region has SiGe material doped with phosphorus or arsenic.

During operation, the PN is reverse biased by applying bias to the electrodes. This reverse bias across the PN junction results in carrier depletion, thereby producing the desired dispersion effect.

In some embodiments, the field is created effectively from a line at the base of the waveguide on one side (in other words at the base of the trench) and across the opposing side of the waveguide. This ensures that the optical mode is effectively in contact with a smaller doped area and optical loss is consequently less. For an exemplary 50 μm modulator length, simulation results show that the optical loss is reduced by about 1.0 dB comparing with where both sidewalls are doped (as in U.S. Pat. No. 8,346,028).

In some embodiments, doping is on either side of a waveguide but not in the exposed side wall of the waveguide. A structure is disclosed by Dawnay et al (U.S. Pat. No. 6,584,239) except that (a) the material is Si (not SiGe) and the effect is electro-optic and not FK and (b) Dawnay discloses an etch to the depth of the oxide layer whereas, in this invention the oxide layer is not exposed.

In the field of germanium photodetectors many structures have been described. However, many of these are vertical stack junction structures which are often more costly to manufacture than horizontal structures and may not be suitable to integrate with Si waveguide on SOI platform, or they are structures where light evanesces from the light transmitting waveguide into a receiving germanium region and are therefore less efficient. The photodetector structures of this invention are structures where silicon/germanium is fabricated into the waveguide and the control and monitoring electrodes are lateral—in other words a horizontal structure. Vivien et al (Optics Express Vol 15, No 15, (2007) p9843) report a germanium photodetector that has MSM (metal-semiconductor-metal) structure in which there is no PN junction (or PIN junction) and no waveguide in the germanium sensing material, therefore, the photodetector of Vivien et al has the disadvantages of high bias voltage and low responsivity.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will be appreciated and understood with reference to the specification, claims, and appended drawings wherein:

FIG. 1A shows an example of a waveguide optical device for use with the present invention;

FIG. 1B shows an example of a waveguide optical device for use with the present invention;

FIG. 1C shows an example of a waveguide optical device for use with the present invention;

FIG. 1D shows an example of a waveguide optical device for use with the present invention;

FIG. 2A shows an example of a waveguide platform suitable for forming part of the modulator of the present invention;

FIG. 2B shows an example of a waveguide platform suitable for forming part of the modulator of the present invention;

FIG. 2C shows an example of a waveguide platform suitable for forming part of the modulator of the present invention;

FIG. 3A shows a schematic example of a plurality of electrodes for use with the present invention;

FIG. 3B. shows a schematic example of a plurality of electrodes for use with the present invention;

FIG. 3C shows a schematic example of a plurality of electrodes for use with the present invention;

FIG. 3D shows a schematic example of a plurality of electrodes for use with the present invention;

FIG. 3E shows a schematic example of a plurality of electrodes for use with the present invention;

FIG. 4 shows a schematic example of a waveguide modulator according to the present invention;

FIG. 5 shows a schematic example of a waveguide modulator according to the present invention;

FIG. 6A shows a schematic example of a waveguide modulator according to the present invention;

FIG. 6B shows a schematic example of a waveguide modulator according to the present invention;

FIG. 7A shows a schematic example of a waveguide modulator according to the present invention;

FIG. 7B shows a schematic example of a waveguide modulator according to the present invention;

FIG. 8A shows a schematic example of a waveguide modulator according to the present invention;

FIG. 8B shows a schematic example of a waveguide modulator according to the present invention;

FIG. 8C shows a schematic example of a waveguide modulator according to the present invention;

FIG. 9 shows an example of a waveguide optical device for use with the present invention;

FIG. 10A shows parameters used to model the waveguide device of FIG. 9;

FIG. 10B shows parameters used to model the waveguide device of FIG. 9;

FIG. 11A shows modelled waveforms for the waveguide of FIG. 9;

FIG. 11B shows modelled waveforms for the waveguide of FIG. 9;

FIG. 11C shows modelled waveforms for the waveguide of FIG. 9;

FIG. 11D shows modelled waveforms for the waveguide of FIG. 9;

FIG. 12 shows an example of a waveguide optical device for use with the present invention;

FIG. 13A shows parameters used to model the waveguide device of FIG. 12;

FIG. 13B shows parameters used to model the waveguide device of FIG. 12;

FIG. 14A shows modelled waveforms for the waveguide of FIG. 12;

FIG. 14B shows modelled waveforms for the waveguide of FIG. 12;

FIG. 14C shows modelled waveforms for the waveguide of FIG. 12;

FIG. 14D shows modelled waveforms for the waveguide of FIG. 12;

FIG. 15 shows an example of a waveguide optical device for use with the present invention;

FIG. 16A shows parameters used to model the waveguide device of FIG. 15;

FIG. 16B shows parameters used to model the waveguide device of FIG. 15;

FIG. 17A shows modelled waveforms for the waveguide of FIG. 15;

FIG. 17B shows modelled waveforms for the waveguide of FIG. 15;

FIG. 17C shows modelled waveforms for the waveguide of FIG. 15;

FIG. 17D shows modelled waveforms for the waveguide of FIG. 15;

FIG. 18 shows an example of a waveguide optical device for use with the present invention;

FIG. 19A shows parameters used to model the waveguide device of FIG. 18;

FIG. 19B shows parameters used to model the waveguide device of FIG. 18;

FIG. 20A shows modelled waveforms for the waveguide of FIG. 18;

FIG. 20B shows modelled waveforms for the waveguide of FIG. 18;

FIG. 20C shows modelled waveforms for the waveguide of FIG. 18;

FIG. 20D shows modelled waveforms for the waveguide of FIG. 18;

FIG. 21 shows an example of a waveguide optical device for use with the present invention;

FIG. 22A shows parameters used to model the waveguide device of FIG. 21;

FIG. 22B shows parameters used to model the waveguide device of FIG. 21;

FIG. 23A shows modelled waveforms for the waveguide of FIG. 21;

FIG. 23B shows modelled waveforms for the waveguide of FIG. 21;

FIG. 23C shows modelled waveforms for the waveguide of FIG. 21;

FIG. 23D shows modelled waveforms for the waveguide of FIG. 21;

FIG. 24 shows an example of a waveguide optical device for use with the present invention;

FIG. 25A shows parameters used to model the waveguide device of FIG. 24;

FIG. 25B shows parameters used to model the waveguide device of FIG. 24;

FIG. 26A shows modelled waveforms for the waveguide of FIG. 24;

FIG. 26B shows modelled waveforms for the waveguide of FIG. 24;

FIG. 26C shows modelled waveforms for the waveguide of FIG. 24;

FIG. 26D shows modelled waveforms for the waveguide of FIG. 24;

FIG. 27 shows an example of a waveguide optical device for use with the present invention;

FIG. 28A shows parameters used to model the waveguide device of FIG. 27;

FIG. 28B shows parameters used to model the waveguide device of FIG. 27;

FIG. 29A shows modelled waveforms for the waveguide of FIG. 27;

FIG. 29B shows modelled waveforms for the waveguide of FIG. 27;

FIG. 29C shows modelled waveforms for the waveguide of FIG. 27;

FIG. 29D shows modelled waveforms for the waveguide of FIG. 27;

FIG. 30 shows an example of a waveguide optical device for use with the present invention;

FIG. 31A shows parameters used to model the waveguide device of FIG. 30;

FIG. 31B shows parameters used to model the waveguide device of FIG. 30;

FIG. 32A shows modelled waveforms for the waveguide of FIG. 30;

FIG. 32B shows modelled waveforms for the waveguide of FIG. 30;

FIG. 32C shows modelled waveforms for the waveguide of FIG. 30;

FIG. 32D shows modelled waveforms for the waveguide of FIG. 30;

FIG. 33 shows an example of a waveguide optical device for use with the present invention;

FIG. 34A shows parameters used to model the waveguide device of FIG. 33;

FIG. 34B shows parameters used to model the waveguide device of FIG. 33;

FIG. 35A shows modelled waveforms for the waveguide of FIG. 33;

FIG. 35B shows modelled waveforms for the waveguide of FIG. 33;

FIG. 35C shows modelled waveforms for the waveguide of FIG. 33;

FIG. 35D shows modelled waveforms for the waveguide of FIG. 33;

FIG. 36 shows an example of a waveguide optical device for use with the present invention;

FIG. 37A shows parameters used to model the waveguide device of FIG. 36;

FIG. 37B shows parameters used to model the waveguide device of FIG. 36;

FIG. 38A shows modelled waveforms for the waveguide of FIG. 36;

FIG. 38B shows modelled waveforms for the waveguide of FIG. 36;

FIG. 38C shows modelled waveforms for the waveguide of FIG. 36;

FIG. 38D shows modelled waveforms for the waveguide of FIG. 36;

FIG. 39 shows a further example of a Mach-Zehnder waveguide modulator according to the present invention;

FIG. 40 shows a further example of a Mach-Zehnder waveguide modulator according to the present invention;

FIG. 41 shows an example of a PN junction waveguide optical device for use with the present invention;

FIG. 42 shows a further example of a PN junction waveguide optical device for use with the present invention;

FIG. 43 shows an example of a Mach-Zehnder modulator comprising PN junctions regions;

FIG. 44 shows an example of an alternative Mach-Zehnder modulator comprising PN junction regions;

FIG. 45 shows an example of yet another Mach-Zehnder modulator comprising PN junction regions;

FIG. 46 shows an example of an arrangement of a plurality of electrodes relative to the active regions of the junctions for use with Mach-Zehnder modulators according to the present invention;

FIG. 47 shows another example of an arrangement of a plurality of electrodes relative to the active regions of the junctions for use with Mach-Zehnder modulators according to the present invention; and

FIG. 48 shows an example of an arrangement of a plurality of electrodes relative to the active regions of the junctions for use with Mach-Zehnder modulators according to the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a Mach-Zehnder waveguide modulator provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.

In FIG. 1A is shown a portion of a SOI chip. This has a silicon base (101) with an insulating layer of silicon dioxide (102). A cavity has been etched into the silicon layer above the oxide and intrinsic SiGe (104) has been grown in a region and etched to create an upstanding waveguide (as shown in FIG. 1). The SiGe is typically grown upon a residual thin (200+/−100 nm) layer of silicon (103) but can also be grown directly upon the silicon dioxide layer (102). Typically such an active waveguide would be contiguously connected to a passive Si waveguide, providing light to the active region and carrying light from the active region. In order to dope appropriately, the waveguide is masked in appropriate steps to allow doping to be applied as shown at 105 for p++ and 106 for n++. The p++ doping is in the sidewall (110) to form a PN junction between region 105 and region 106 in the optical waveguide. The PN junction overlaps with the optical mode to have a strong FK effect in the waveguide. It may be advantageous, for example to control the electric field or to reduce optical loss, to create non-uniform doping in the doped regions 105 and/or 106. For example, up to 4 levels of doping could be created within regions 105 and/or 106. The contact electrodes are 110.

It is preferred that the p++ doping is in the sidewall and the n++ doping in the lateral surface and not on a sidewall because the intrinsic SiGe (104) is typically lightly n doped (typically at 1×10¹⁵ to 1×10¹⁶ cm⁻³). Thus the pn junction for a device of this type with a p doped side wall will be in the region of the optical mode and will be a more effective modulator than if the reverse arrangement of doping were used with lightly p doped intrinsic active semiconductor. However, the reverse arrangement of doping is within the scope of this invention and would be effective when the so-called intrinsic SiGe were lightly p doped. However, a lightly p doped region will typically have a higher concentration of dopant than the intrinsic region (lightly n doped) and an n++ wall has higher optical absorption than p++ doped wall.

Waveguides of this invention are optionally of approximately 3 μm height above the oxide layer but a smaller dimension, for example 0.5 μm is also possible.

In a further embodiment of this invention, the doped waveguide wall may be made of doped silicon. This has the advantage of reducing the optical loss even more. Furthermore, the doped regions of the waveguide slab may also be made of silicon. In FIG. 1B is shown an example where the intrinsic SiGe is 104. Region 107 is a p doped silicon in the wall 111 of the waveguide and region 108 is n doped silicon. Alternatively region 108 may be n doped SiGe. The doping may be in the opposite sense with n doping in the silicon side wall and p doping in the slab region. Furthermore the intrinsic SiGe may be lightly p doped rather than the normal lightly n doped material. The contact electrodes are 110. The means of manufacturing such structures would be similar to that of the device of FIG. 7.

Yet further embodiments of the present invention are illustrated in FIGS. 1C and 1D. In FIG. 1C the wall regions of the waveguides are shown as 111 and 112. As shown, region 106 is n doped active material including a doped wall region 112. Wall 111 is made of silicon as is the connected slab region and this entire region 107 is p doped. As in earlier embodiments the doping may be in the opposite sense and the intrinsic region, normally very lightly n doped, may be p doped.

In FIG. 1D is shown a structure similar to that of FIG. 1C but where the doped region 106 does not extend to the top of the waveguide wall 112. Again the doping may be arranged in the opposite sense. Furthermore, the wall which is entirely doped may be the wall made of intrinsic active semiconductor and the partially doped wall may be the silicon wall. The dopant implanted into either of the vertical regions may not extend to the very top of the vertical region. There may be an undoped section of the vertical region which is substantially undoped. The undoped section may be located above a doped section of the vertical region. Such a substantially undoped region may have the same doping as any intrinsic doping of the central region. The extent/height of the doped section in the first and second vertical regions may be different (for example, FIG. 1D). The height of the doped section in each the first and second vertical regions may be asymmetrical about the central region.

In some embodiments as described herein, one wall of the waveguide is partially undoped. It is meant that the dopant may only extend to a certain height up the wall from the base. There may therefore be a vertical section of the wall that is doped, and a vertical section of the wall above the doped section which is substantially undoped. This undoped region or section of the wall may have no, or substantially no dopant added. There may be some intrinsic dopant in the material of the wall which is consequently present in the undoped section.

In accordance with invention 1, the vertical region may be a region of the central region located at an edge of the central region. The presence of the dopants in the vertical regions (which may be only in one vertical region) therefore define the locations of the vertical regions within the central region. The vertical regions, the lateral regions, and central region may be composed of the same material, which may be for example SiGe (or Ge for examples outside of the scope of the current claims) or any suitable alternative known to those skilled in the art. The lateral regions, vertical regions and the central region could be integral with one another. The lateral regions, vertical regions and the central region may be formed from a single slab of material, which is then etched away to form the upstanding central region. In this case the lateral regions and the vertical regions may be selectively doped by masking during the fabrication process.

Alternatively, the vertical region may be a distinct element which abuts the common region. Where the vertical region is a distinct element, the distinct element may be composed of a different material from the active material of the central region. For example, the central region may be composed of SiGe (or Ge), and the vertical regions and the lateral regions may be composed of Si.

It is also possible, that the doped vertical region and doped lateral region on a first side of the waveguide be composed of doped SiGe (or doped Ge), and the doped lateral region and the vertical region on the opposite be composed of doped Si, for example.

During operation of the active optical waveguides, electrodes attached to the doped lateral regions may be used to apply electrical energy to the vertical doped regions. The electrical energy application may be in the form of a bias between the first and second lateral doped regions. Each vertical doped region may be integral with the adjacent doped lateral region, and as a consequence, any electrical energy applied to the lateral regions is also applied to the doped vertical regions. Consequently, a corresponding electrical bias may be generated between the doped vertical regions. This application of electrical energy forms an electrical field in the active material through which light passes. Such an electric field in the active material can be formed without generating a significant electrical current through the active material.

The active material may be a material in which the Franz-Keldysh effect occurs in response to the application of the bias to the doped regions, and the consequent electric field which is formed. The Franz-Keldysh effect is a change in optical absorption and/or optical phase of light passing through the central active region. The Franz-Keldysh effect is caused by the presence of an electric field in the active material of the central active region. For instance, the Franz-Keldysh effect allows an electron in a valence band to be excited into a conduction band by absorbing a photon, even though the energy of the photon which is absorbed is smaller than the band gap. In other words, the active material may have a slightly larger band gap energy than the photon energy of the light to be modulated. The application of the bias between the doped regions, and, and the consequent electric field formed across the active central region, lowers the absorption edge and makes absorption of a photon possible. The hole and electron carrier wavefunctions overlap when the electric field is applied and generation of an electron-hole pair becomes possible. As a result, the central active region can absorb light signals received by the central active region. Increasing the electrical field may increase the amount of light absorbed by the central region. Accordingly, electronics that are connected to the electrodes can be used to tune the electrical field so as to tune the amount of light absorbed by the active material of the central region. As a result, the electronics can modulate the electrical field in order to modulate the amplitude of the outgoing light signal.

Although outside of the scope of the claims of this invention, the active optical device of each waveguide arms of the modulators of the present invention may also be used as a photodetector by applying a reverse bias across the common central region, by suitable application of electrical charge to the electrodes, which are electrically connected to the doped vertical region(s) via the doped lateral regions. This reverse bias means that at least some of the light passing through the central region will be absorbed by the central common region. This absorption may cause a current to flow through the central common region between the doped regions. Measurement of such a current flow may then be indicative of the receipt of a light signal. The intensity of the current generated may depend on the power or intensity of the light received (the light passing through the central region). In this way the active optical waveguide can be configured to operate as a photodetector.

Further embodiments of a waveguide modulator will now be described in relation to FIGS. 2A-2C, typically with waveguide ridge width of less than 1 μm and preferably less than 0.6 μm. Also typically, the height of the waveguide ridge above the oxide layer is arranged such that more than 50% of the power of the optical mode is located below the top of the slab region of the SiGe (or Ge or other appropriate semiconductor material) on either side of the ridge.

In FIGS. 2A and 2B the numbers correspond to the numbers in FIG. 1. The upstanding rib waveguide is a single mode waveguide with PIN junction. The central part (204) (iSiGe) of the waveguide has dimensions around 0.5×0.5 μm. The upstanding height is 0.2 micron (approx.). The two sides of the slab waveguide are doped with n and p types, so there is an electric field in the central iSiGe when the PIN is reverse biased, therefore the FK effect may be induced in the central part to modulate the light. In the structure of FIG. 2A lighter doped regions are inserted between the active intrinsic SiGe waveguide and the more heavily doped regions (to reduce the optical loss). The lightly p doped region is 207 and the lightly n doped region is 208. The more heavily doped regions are 205 (p) and 206 (n). Compared with structure in FIG. 2A, the structure in FIG. 2B will have bigger optical loss but smaller driving voltage and higher speed. Both structures can be used to make photodetectors. The advantages of these structures are 1) small size; 2) high speed; 3) low driving voltage thus low power consumption.

Such a structure may be created on an SOI platform by etching a cavity in the silicon close to the oxide layer, growing SiGe of the appropriate composition in the cavity, planarising the surface of the wafer using chemical mechanical polishing (CMP) or similar and then etching the waveguides in the silicon and SiGe material to form a contiguous waveguide through the two materials. The SiGe at the sides of the SiGe waveguide is then selectively doped to provide the electrical contact regions and the regions creating the electric field across the optical waveguide.

A further embodiment is shown in FIG. 2C. Here is shown in cross section the small size waveguide as in the above description but with doping on one side only of the upstanding ridge. This has similar advantages to the structures of FIG. 1 with the added benefits of small size, especially speed of operation and lower power consumption. Again it is preferable that the side of the upstanding ridge is p doped (contiguous with the p doping of the slab region) but the opposite arrangement is quite practical—where the side of the n doping in the slab has the side of the waveguide n-doped. It is often useful to grade the doping so that the region closest to the optical mode has a lower dopant concentration. Electrical contacts are shown as 210.

These embodiments are particularly advantageous in creating high speed modulators where the active waveguide (SiGe) of the modulator is of the smaller dimensions. In this case the small waveguide SiGe modulator would be integrated onto an SOI platform having typically 3 um waveguides and the modulator would be connected to the 3 um waveguides by waveguide tapers constructed in the silicon.

Mach-Zehnder (MZ) modulators constructed in waveguides on an SOI platform may be made with the structures in either FIG. 1 or 2 as the active areas in their arms. Such structures are illustrated in FIGS. 3, 4, 5, 6 and 7.

FIG. 4 shows a cross section through the waveguide arms of a Mach-Zehnder modulator of one embodiment of this invention. The waveguides are made of intrinsic SiGe (404) upon a silicon on oxide base. Here the waveguides are made in a single cavity of SiGe and the p doped side walls (405) are shown on the inner sides and a DC bias contact is placed centrally to provide a common bias for the two active waveguides. The n doped regions (406) are positioned on the outer sides and support RF contacts.

The common doped region may be located between the waveguide arms of the Mach-Zehnder modulator. This common region may comprise the similarly doped lateral regions of each of the active optical waveguide comprised in each arm. For example, the p-doped lateral region of an optical waveguide of the first arm may be integral with the p-doped lateral region of an optical waveguide in the second arm. In this example, there is a p-doped common region located between the arms. There may also be an n-doped lateral region on an opposite side of the each optical waveguide to the common p-doped region. In such an example embodiment, the arrangement of the doped regions of the first optical waveguide is a mirror-image of the doped regions of the second optical waveguide. The reverse arrangement is equally an embodiment of the present invention. In the reverse example, the common region would be n-doped and the outer regions of each waveguide would be p-doped.

As a further refinement of the present invention, it may be that the at least partially doped vertical region of each of the active optical waveguides in each arm is integral with the common region located between the arms. Conversely, it is equally possible that the at least partially doped wall of each of the active optical waveguides is integral with the lateral region on the outer sides of the active optical waveguides.

FIG. 5 shows an alternative arrangement of equal merit where the n doped region is centrally located and the p doped regions are on the outer sides of the Mach-Zehnder waveguide structure.

In respect of the modulators of FIGS. 4 and 5, in accordance with the teaching of Invention 1, the side wall doping can be n, but p doping of the side walls is to be preferred.

In FIG. 6 are shown upstanding waveguides in SiGe, Ge or similar material where the waveguides are of the type described in relation to FIG. 2. Here they are assembled into Mach-Zehnder structures. The SiGe waveguides are fabricated upon an SOI chip. Then doped regions are numbered in accordance with the earlier figures and it will be clear that intermediate lightly doped regions may be used in accordance with the teaching of FIG. 2A.

In FIG. 7 are shown SiGe channel waveguides based on SOI layer (SiGe channel width 0.2-0.5 μm, channel height 0.5-2.6 μm, width of Si side-cladding 0.1-0.2 μm and height of Si side-cladding 0.5-2.6 μm depending on the waveguide platform). A channel (or slot) is created in an SOI wafer and filled with SiGe. After the wafer has been planarised, waveguides are then etched in the silicon to create silicon waveguides contiguous with waveguides with a SiGe central part (E) but with some silicon remaining to form the side-cladding walls of the waveguide. Doping is then applied to the silicon layers (F and G) on either side of the waveguide, taking care not to dope the SiGe or Ge. The SiGe waveguide is contiguous with passive silicon waveguides acting to carry light to the active region and to carry it away. Portions of the lateral regions (H and J) are also doped so that convenient electrical contacts can be made at metallised areas (not shown). The advantage of this structure is that a lower voltage is required for a given modulation depth (compared with U.S. Pat. No. 8,346,028) with lower optical loss since optical mode is mainly confined in the SiGe channel area with less amplitude in the Si side cladding area which is doped.

For all of the cross-section structures of Mach-Zehnder modulators shown in FIGS. 3, 4, 5, 6 and 7, various arrangements may be constructed along the length of the waveguide. In FIG. 3, some of these arrangements are illustrated. FIG. 3A illustrates a lumped electrode created in a SiGe region (cavity) built upon an SOI wafer. The light incident at the input x of the device is split, typically using a multi-mode interference (MMI) device to divide the light between the arms of the Mach-Zehnder modulator. Other light splitting means will be known to one skilled in the art. Each of the arms is created through a region of SiGe. There is a central DC bias electrode, typically dispose on the surface of the horizontal doped region and the RF drive for the electrodes is achieved through metal contacts placed on the horizontal portions of the lateral horizontal doped regions.

FIG. 3B shows an example of cascade lumped electrodes (series of electrodes) fabricated in a SiGe region (cavity) of the chip such that high speed modulation may be achieved. FIG. 3C shows an example where there are multiple regions (cavity) of SiGe, each having appropriate electrodes. The advantage of such structures is that they have lower power consumption, but the disadvantage is that they are more complicated to drive and control.

FIG. 3D shows an example of a traveling wave electrode fabricated in a SiGe region (cavity) of the chip such that high speed modulation may be achieved. FIG. 3E shows an example where there are multiple regions of SiGe a modulator has traveling wave electrodes. The traveling wave electrodes are not necessary the same structures shown in FIGS. 3D and 3E which are just examples. The definition of traveling wave electrodes in this patent is any electrodes that are terminated with resistors (typical 50 ohm value). The advantages of traveling wave electrodes are to have high speed, low driving voltage and large extinction ratio, but the disadvantages of traveling wave structures are that they tend to be longer (100 μm to 5 mm), so occupy more space than lumped electrodes, and they consume more power than lumped electrodes due to the 50 ohm termination resistance with existing driving techniques.

Typically, the Mach-Zehnder modulators of this invention would be fabricated on a silicon-on-insulator (SOI) chip. FIG. 3 illustrate a MZ interferometer in which the input signal is received at an input waveguide to a multi-mode interference coupler (MMI) which divides equally the input light into the two arms of the MZ interferometer structure. The central region of the MZ modulator is a silicon/germanium cavity. At the very least the optical waveguides in this region are made of silicon/germanium of an appropriate stoichiometric ratio to cover interested wavelength range. As shown in FIGS. 4, 5, 6 and 7, the one side (inner side) of the waveguides are p or n doped and the other sides are oppositely doped (that is n or p doped respectively). The MZ is DC biased at the central electrode and RF signals are applied to the arms of the interferometer at electrodes contacted at the two sides. When the MZ modulator of FIG. 4, 5, 6 or 7 is operated in a push-pull mode, in one arm the DC electric field and RF electric field are in the same direction; but in the other arm the DC electric field and the RF electric field are in opposite direction, the phase change in the two arms induced by the electric field change has the same amplitude but opposite direction due to the carrier plasma dispersion effect. Therefore, the total phase change in the two arms is doubled, which reduces the drive voltage in half.

Further embodiments are disclosed in relation to FIG. 8, which illustrate a Mach-Zehnder (MZ) interferometer constructed with MMI couplers on an SOI chip. FIG. 8A is a plan view of one possible structure. As with previous embodiments, the incoming light (at x), may be divided and recombined by various means known to one skilled in the art.

In this invention the arms of the MZ are isolated from each other—in other words there is no common bias. Each of the arms is effectively a phase shifter fabricated in a SiGe cavity (enclosed in dotted line in FIG. 8A) in the silicon chip. As shown in FIG. 8B the waveguides of the structure of FIG. 1 are in the driven region of the modulator and they are made of SiGe of an appropriate stoichiometric ratio to cover the wavelength of interest. In a preferred mode of operation, to each arm of the modulator is applied an RF modulating signal at the electrodes marked 910, where the two RF signals are out of phase—effectively producing a push-pull drive for the MZ modulator.

In this embodiment any of the waveguide structures of FIG. 1 (FIG. 8B) and 2 may be used as well as SiGe waveguides with doped Si sidewalls (FIG. 8C). In addition, as for Invention 3, multiple lumped electrodes or traveling wave electrodes with single SiGe cavity and multiple lumped electrodes or traveling wave electrode with multiple SiGe cavities may be used.

It will be appreciated that features of each of the Inventions described herein may be combined with one another in arrangements not explicitly claimed as such.

Mach-Zehnder Modulator

There is here described a Mach-Zehnder modulator (MZM). The MZM has an optical splitter, two arms which may be substantially parallel, and an optical combiner. Each arm comprises an optical waveguide in which light is guided. Each of the waveguide arms can have one or more active optical sectors. In one example, each of the two arms of the MZM will generally have more than one active optical sector, but it is equally possible to have a single active optical sector in each arm of the MZM.

A sequence of active optical sectors may be provided in each waveguide arm, light being guided along the waveguides constructed though each active sector. The sequence of active optical sectors along each arm may be generally aligned.

Each waveguide in an active optical sector in the MZM may have a doped wall on each side. Each waveguide in an active optical sector may have a first wall which is doped to the full height of the wall. In other, words the whole of the wall on the first side of the central region may be doped. At the same time, the second wall opposite to the first wall may be at least partially doped. Partial doping of this wall may equate to the dopant only extending a certain fraction up the height of the second wall. There may equally be no doping in the second wall. The result of such a doping scheme may be that the distribution of doping in the first and second walls of the active optical waveguide sector is asymmetrical in cross-section. For example, the first wall may be a p-doped to a first height, and the second wall may be n-doped to a second height and the first and second heights may not be the same. Equally, the first wall may be n-doped to a first height, and the second wall may be p-doped to a second height. The first and second heights may not be the same. Equally, the first wall may be a p-doped to a first height, and the second wall may be substantially undoped. Equally, the first wall may be n-doped to a first height, and the second wall may be substantially undoped. When the second wall is undoped, there may be a lateral doped region adjacent to the second wall which is doped. The doping at an edge of this lateral doped region may be in contact with the active central region.

The sense of the doping will be opposite in opposing walls (or a lateral region in the case that one of the walls is undoped) on either side of the central region, one wall will be p-doped and the opposite wall (or lateral region) will be n-doped for a given active optical waveguide sector. Extending outwards from the base of each doped wall may be a doped lateral region. The doping of each lateral region will be of the same sense as the doping in the doped wall from which it extends. In this way, each doped wall and adjacent doped lateral region together may have a cross-section which is “L”-shaped. Advantageously, electrical connections may then be formed on the lateral regions, which are electrically connected (via the doping) to the vertical walls (which may be partially doped). The lateral regions may provide a convenient surface on which to form electrical connections. In this way, an electrical signal which is applied to an electrode which is attached to a lateral doped region is transmitted, because of the common doping, to the adjacent vertical doped region.

The waveguide arms of the MZM are generally parallel and there will exist an inner region between the arms. A pair of active optical waveguide sectors, a first active optical waveguide sector comprised in the first arm and a second active optical waveguide sector comprised in a second arm that are adjacent with one another, will both be adjacent to the inner region located between the arms of the MZM. The inner region may be also be doped. The doping of the inner region may be the same as the doping in the adjacent lateral regions of each of the adjacent active optical waveguide sectors. It may be that the adjacent lateral regions of the active optical waveguide sectors meet one another in the inner region. The inner region may effectively join the adjacent lateral regions of each of the adjacent active optical waveguide sectors. The material of the adjacent lateral regions in the inner region may be integral with one another, forming an inner doped region. A DC bias or an electrical signal which is applied to the inner doped region is consequently applied to the inner doped lateral regions (which may be integral with one another in the inner region). Because the doped lateral regions in the inner region are each connected to the inner doped vertical regions, any electrical signal applied to the inner lateral regions is also applied to the vertical doped regions adjacent to the inner region. In this way an electrical connection providing an electrical signal to the inner region is applied similarly to the doped vertical regions of adjacent active optical waveguide sectors in each arm.

For an active optical waveguide to cause a change the phase of light passing through it, an electric field must be formed in the central region. To form this field an electrical connection must be applied to both of the doped opposite sides of each active optical waveguide sector in the MZM. As described above, this electrical connection may conveniently be applied to the lateral doped regions adjacent, and electrically connected to, the vertical regions. As well as a common electrode provided to apply a common bias to the inner doped walls of an adjacent pair of active optical waveguide sectors, there may also be at least one electrical connection applied to each of the outer doped walls of the adjacent pair of active optical waveguides. To apply the electrical signal to each of the outer doped vertical walls, the electrical connection may be applied to the respective doped lateral regions, which extend from the doped vertical region(s). In this way, for each active optical waveguide sector, by applying a bias or a reverse bias between the common region and an outer doped wall, an electric field is formed within the central region. The electrical connection to the outer doped regions may be an RF signal, which provides the modulation signal for the optical phase change to be applied. This electric field may then give rise to a phase change in the light passing through the central region.

For an adjacent pair of active optical waveguide sectors, one waveguide sector comprised in the first arm and the other active optical sector comprised in the second arm, there may be two outer doped walls and two inner doped walls (one inner wall and one outer wall either side of each of the active optical waveguide sectors). The inner doped walls may be connected to inner doped lateral regions to form a common doped region between the arms of the MZM. The two outer walls will have the same doping sense as one another, for example both outer doped walls may be p-doped or both outer walls may be n-doped. The inner walls and the common region will be doped in the opposite sense to the outer walls. For example, where the outer walls are both p-doped, the inner walls may both be n-doped. Conversely, where the outer walls are both n-doped, the inner walls may both be p-doped. As described above, it may be that there is only partial doping in one of the walls for each of the active optical waveguide sectors. Equally, it may be that there is no doping in one of the walls for each of the active optical waveguide sectors. This equates to an asymmetrical doping cross-sectional profile for each of the pairs of active optical waveguide sectors.

Where each arm of the MZM includes a plurality of active optical waveguide sectors, this plurality can be described as a plurality of adjacent pairs of active optical waveguide sectors. Each pair of active optical waveguide sectors can be configured as described above. Where a number of pairs are comprised along the length of the MZM, the central region of all of the active optical sectors comprised in each arm will be aligned to allow the containment of light. Each active optical waveguide sector may be used to impart a different magnitude of phase change to the light. The cross-sectional profile of the dopants across a pair of active optical waveguide sectors may be symmetrical about an axis passing through the common region located between the active optical waveguides. Equally, the cross-sectional profile of the dopants across a pair of active optical waveguide sectors may be asymmetrical. In the asymmetrical case for each pair the dopants' profile in each active optical waveguide sector may be the same. The total phase change applied to the light in an arm of the MZM may be the sum of the phase changes applied by the active optical waveguides comprised in that arm. Each active optical waveguide sector may be used to impart a different magnitude of phase change to the light.

Where there is a plurality of pairs of active optical waveguide sectors along the length of the MZM, the common regions of the pairs may be aligned with one another along the length of the MZM. A common signal may be applied to the aligned central regions. This may be achieved with a common electrode rail which runs along the length of the common regions and is electrically connected to each of the common regions. Alternatively, there may be provided a sequence of individual electrical connections to the common regions, and these electrical connections may be electrically connected to each other, for example using a wire connection. In both examples, the same electrical signal may be applied to all of the common regions.

The outer vertical regions and the outer lateral regions of a plurality of pairs of active optical waveguide sectors may also be aligned along the length of the MZM. Along a sequence of outer lateral regions along each side of the MZM there may be a separate lumped electrode provided on each lateral region. In this case each waveguide sector can have a bias between the common electrical connection to the common region of the between the arms of MZM and the outer lumped electrode. The phase change of the light contained within the first arm may be opposite to that in the second arm. In this way the MZM may be configured to be operated in a “push-pull” mode.

Alternatively, a rail electrical connection may be connected to a plurality of aligned outer doped regions on each side of the MZM. This rail may form a travelling wave electrode. The outer rail on the first side of the MZM may be terminated with the outer rail on the opposite side of the MZM.

The advantages of various embodiments of the present invention can be further understood by the mathematical models presented in FIGS. 9 to 38.

Further embodiments of Mach-Zehnder modulators having PIN junctions are described below in relation to FIGS. 39 and 40. This Mach-Zehnder modulators operate as push-pull Mach-Zehnders with forward bias.

FIG. 39 depicts a Mach-Zehnder waveguide modulator wherein the left and right waveguides are formed from SiGe, the SiGe portion laying on top of a silicon-on-insulator (SOI) base. For each of the left and right waveguides, the junction region is a PIN junction, the PIN region comprising a first doped region in the form of an n doped region (N), 391 and a second doped region in the form of a p doped region 392 a, 392 b.

For each waveguide, the N doped region extents vertically along the side wall of the waveguide, laterally along the upper surface of the waveguide, and laterally at the base of the ridge, away from the waveguide. The N doped regions of the two waveguides meet to form a shared N doped region between the left arm and right arm of the Mach-Zehnder modulator. A concentrated sub-region N+ is located within the N doped region, between the two waveguide.

The p doped region for each waveguide consists of a first p doped sub region (P), located within the SOI layer, and a second more concentrated p+ sub region (P+), in electrical contact with the p sub region, the p+ sub region being located within the SiGe material.

A plurality of electrodes are located in contact with the n+ and p+ sub regions. Since the n+ sub region is shared, a central common electrode is also shared between the two waveguides.

The embodiment shown in FIG. 40 differs from that of FIG. 39 in that there is no shared doped region. In FIG. 39, the waveguide junctions for respective arms can be thought of as mirror images of each other. For FIG. 40, this is not the case. Instead, the central region between the two waveguides comprises the N doped region of one waveguide and the p doped region of the other waveguide, separated by an insulating gap. This results in an extra n+ region and an extra electrode.

Examples of Mach-Zehnder modulators comprising PN junction regions are described below in relation to FIGS. 41-45. As used herein, a “junction region” of a waveguide if a region of the waveguide that contains a semiconductor junction. For example, a “PIN junction region” of a waveguide is a region of the waveguide that contains a PIN junction.

FIGS. 41 and 42 show two different designs for optical PN devices that could be used to form the PN junction region of each waveguide of the Mach-Zehnder waveguide modulator. In the embodiment of FIG. 41, the N doped region and P doped region each extend vertically along the entire height of one side of the waveguide. The meet at a vertical or substantially vertical boundary within the SiGe waveguide.

In the embodiment of FIG. 42, only one of the doped portions extends up to the full height of one of the side walls of the waveguide, in this case the N doped region. At the top of the waveguide, the N doped region extends laterally along the upper surface of the waveguide.

FIGS. 43 and 44 show Mach-Zehnder modulators where the doped junctions are formed from optical devices such as that of FIG. 41. In each of these embodiments, the doped region between the two waveguide arms is shared.

FIG. 45 also shows a Mach-Zehnder modulator formed from devices such as that of FIG. 41. However, in this case, the central region in-between the waveguides includes an n doped region of one waveguide and a p doped region of the other waveguide, the two doped regions being separated by an intrinsic region. In other words, this Mach-Zehnder modulator has arms with two PIN junctions separated. This Mach-Zehnder modulator operates as push-pull with forward bias.

Possible arrangements of the plurality of electrodes relative to the active regions of the junctions for use with Mach-Zehnder modulators are described below in relation to FIGS. 46 to 48.

In the embodiment of FIG. 46, a continuous active region is present. The single continuous active region extends over an area along the length of both waveguides and also within the space between the waveguides. The portion of the waveguides within the active region has either PIN (forward bias) or PN junction (reverse bias); and the electrodes can be either lumped electrodes or traveling wave electrodes.

In the embodiment of FIG. 47, strips of active regions lie transverse or substantially transverse (substantially perpendicular) to strips of electrodes. The strips of electrodes extend along a direction parallel to that of the waveguides. Again, the portions of the waveguides within the active region has either PIN (forward bias) or PN junction (reverse bias); and the electrodes can be either lumped electrodes or traveling wave electrodes

The embodiment of FIG. 48 differs from that of FIG. 47 in that the electrodes are discrete rather than continuous strips. Again, the portion of the waveguides within the active region has either PIN (forward bias) or PN junction (reverse bias); and the electrodes can be either lumped electrodes or traveling wave electrodes

The following numbered paragraphs (paras.) contain statements of broad combinations of the inventive technical features herein disclosed:—

Group A

1A. An active optical waveguide, comprising:

-   -   a central waveguide region composed of an active material;     -   a first vertical region on a first side of the central region,     -   the first vertical region being at least partially doped, and;     -   a second vertical region on a second side of the central region         the second vertical region being at least partially undoped or         intrinsic, and;     -   a first doped lateral region adjacent to the first vertical         region,     -   a second doped lateral region adjacent to the second vertical         region;     -   wherein the doping in the second lateral region is of an         opposite sense to the doping in first vertical region.

2A. An active optical waveguide according to any preceding para, wherein:

-   -   the first vertical region is composed of the active material or         Si.

3A. An active optical waveguide according to any preceding para, wherein:

-   -   the second vertical region is composed of the active material or         Si.

4A. An active optical waveguide according to any preceding para, wherein:

-   -   the first lateral region is composed of the active material or         Si.

5A. An active optical waveguide according to any preceding para, wherein:

-   -   the second lateral region is composed of the active material or         Si.

6A. An active optical waveguide according to any preceding para, wherein:

-   -   the active material is Ge or SiGe.

7A. An active optical waveguide according to any preceding para, wherein:

-   -   the second vertical region is completely undoped or intrinsic.

8A. An active optical waveguide according to any preceding para, wherein:

-   -   the first vertical region is p-doped and the second lateral         region is n-doped.

9A. An active optical waveguide according to any one of paras 1A-7A, wherein:

-   -   the first vertical region is n-doped, and the second lateral         region is p-doped.

10A. An active optical waveguide according to any preceding para, wherein:

-   -   the doping concentration in the first vertical region is         non-uniform across thickness of the first vertical region.

11A. An active optical waveguide according to any preceding para, wherein:

-   -   the doping concentration in the second lateral region is         non-uniform across a width of the second lateral region.

12A. An active optical waveguide according to any preceding para further comprising:

-   -   at least one electrical contact on the first lateral region,         and;     -   at least one electrical contact on the second lateral region.

13A. An active optical waveguide according to any preceding para, wherein:

-   -   the vertical extent of the doping in the first vertical region         exceeds the thickness of the doping in the second lateral         region.

14A. An active optical waveguide according to any preceding para, wherein:

-   -   the vertical extent of the doping in the first vertical region         exceeds the height of any doping in the second vertical region.

15A. An active optical waveguide according to any preceding para, wherein:

-   -   the central region is intrinsically n-doped.

16A. An active optical waveguide according to any preceding para, wherein:

-   -   a pn-junction is formed within the central region between the         first vertical region and the second lateral region.

17A. An optical waveguide according to any preceding para, wherein:

-   -   the waveguide is located on a silicon on insulator (SOI) base.

18A. An optical device including an active optical waveguide according to any preceding para, wherein:

-   -   the device is an optical modulator.

19A. An optical device including an active optical waveguide according to any preceding para, wherein in use:

-   -   the application of an electric field to the central region,         between the first vertical region and the second lateral region,         gives rise to a Franz-Keldysh (FK) effect in the active         material.

20A. An optical device including an active optical waveguide according to any preceding para, wherein:

-   -   the device is a photodetector.

21A. An optical device according to any one of paras 18A-20A, wherein:

-   -   the device is connected to at least one Si waveguide.

22A. An optical device according to any one of paras 18A-20A, wherein:

-   -   the active material is SiGe, and;     -   the device is connected to at least one Si waveguide.         Group B

1B. An active optical waveguide comprising:

-   -   a central waveguide region being made from an undoped or         intrinsic semiconductor material;     -   a first doped semiconductor region on a first side of the         central waveguide region;     -   a second doped semiconductor region on a second side of the         central waveguide region;     -   wherein the first and second sides of the central waveguide         region are opposite one another;     -   the doping in the first semiconductor region is of an opposite         sense to the doping in the second semiconductor region, and;     -   the central waveguide region has a greater height than either of         the first doped semiconductor region and the second doped         semiconductor region.

2B. An active optical waveguide according to para 1B, wherein:

-   -   the first semiconductor region is p-doped, and;     -   the second semiconductor region is n-doped.

3B. An active optical waveguide according to para 1 or para 2, wherein:

-   -   the first semiconductor region, the second semiconductor region,         and the central waveguide region together form a PIN junction.

4B. An active optical waveguide according to any preceding para, further comprising:

-   -   an intervening lightly p-doped semiconductor region located         between the p-doped semiconductor region and the central         waveguide region;     -   wherein the intervening lightly p-doped semiconductor region has         a lower dopant concentration than the p-doped semiconductor         region.

5B. An active optical waveguide according to any preceding para, further comprising:

-   -   an intervening lightly n-doped semiconductor region located         between the n-doped semiconductor region and the central         waveguide region;     -   wherein the intervening lightly n-doped semiconductor region has         a lower dopant concentration than the n-doped semiconductor         region.

6B. An active optical waveguide according to any preceding para, wherein:

-   -   the central waveguide is composed of Ge or SiGe.

7B. An active optical waveguide according to any preceding para, wherein:

-   -   the central waveguide is composed of Ge or SiGe and the doped         semiconductor regions are composed of Si.

8B. An active optical waveguide according to any one of paras 1B-5B, wherein:

-   -   the central waveguide region, the first semiconductor region,         and the second semiconductor region are all composed of Ge or         SiGe.

9B. An active optical waveguide according to any one of para 4B or para 5B, wherein:

-   -   the central waveguide region, the n-doped semiconductor region,         the p-doped semiconductor region, the lightly p-doped         semiconductor region, and the lightly n-doped semiconductor         region are all composed of Ge or SiGe.

10B. An optical waveguide according to any preceding para, wherein:

-   -   the waveguide is located on a silicon on insulator (SOI) base.

11B. A optical waveguide according to any preceding para, wherein:

-   -   a wall of the central waveguide region adjacent to the p-doped         semiconductor region is p-doped.

12B. A optical waveguide according to any preceding para, wherein:

-   -   a wall of the central waveguide region adjacent to the n-doped         semiconductor region is n-doped.

13B. An optical device including an active optical waveguide according to any preceding para, wherein:

-   -   the device is an optical modulator.

14B. An optical device including an active optical waveguide according to any preceding para, wherein in use:

-   -   the application of an electric field to the central region,         between the first vertical region and the second lateral region,         gives rise to a Franz-Keldysh (FK) effect in the active         material.

15B. An optical device including an active optical waveguide according to any preceding para, wherein:

-   -   the device is a photodetector.

16B. An optical device according to any one of paras 13B-15B, wherein:

-   -   the device is connected to at least one Si waveguide.

17B. An optical device according to any one of paras 13B-15B, wherein:

-   -   the active material is Ge or SiGe, and;     -   the device is connected to at least one Si waveguide.

18B. An optical device according any one of paras 11B and 12B is connected to at least one Si waveguide of a larger size than the central waveguide via at least one Si waveguide spot size converter.

19B. A optical waveguide according to any preceding para, wherein:

-   -   the central region is upstanding from the p-doped region and the         n-doped region by a distance that is less than the height of the         p-doped region or the n-doped region.

Group C: MZ with (optical devices of Group A);

1C. A Mach-Zehnder modulator with two arms, comprising:

-   -   a left arm comprising a left active optical waveguide, and a         right arm comprising a right active optical waveguide;     -   wherein each of the left and right optical waveguides are         according to any para of Group A;     -   wherein the first lateral region of the left optical waveguide         is integral with the first lateral region of the right optical         waveguide in a region between the left and right arms, forming a         common doped region.

2C. A Mach-Zehnder modulator according to para 1C, further comprising:

-   -   a common electrode connected to the common doped region.

3C. A Mach-Zehnder modulator according to para 1C, further comprising:

-   -   a plurality of common lumped electrodes attached to the common         doped region along the waveguide.

4C. A Mach-Zehnder modulator according to para 3C, wherein:

-   -   the plurality of common lumped electrodes are electrically         connected to each other.

5C. A Mach-Zehnder modulator according to any preceding para, further comprising:

-   -   a single left electrode connected to the second lateral region         of the left waveguide, and;     -   a single right electrode connected to the second lateral region         of the right waveguide.

6C. A Mach-Zehnder modulator according to para 5C, wherein:

-   -   the left and right electrodes are each lumped electrodes or         traveling wave electrodes.

7C. A Mach-Zehnder modulator according to para 2C or para 3C, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second lateral region of the left waveguide,         and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second lateral region of the right waveguide;     -   wherein the number of left lumped electrodes and the number of         right lumped electrodes are the same.

8C. A Mach-Zehnder modulator according to para 7C, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the plurality of central electrodes are         contained within a single of active material.

9C. A Mach-Zehnder modulator according to para 3C, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second lateral region of the left waveguide,         and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second lateral region of the right waveguide;     -   wherein the number of left lumped electrodes, the number of         right lumped electrodes, and the number of common electrodes is         the same.

10C. A Mach-Zehnder modulator according to para 9C, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the central electrode are contained         within a single of active material.

11C. A Mach-Zehnder modulator according to any one of paras 7C-9C, wherein:

-   -   the electrodes comprises a plurality of electrode subgroups,         each electrode subgroup comprising:     -   at least one of the plurality of right electrodes;     -   at least one of the plurality of left electrodes, and;     -   at least one of the plurality of central electrodes;     -   each electrode subgroup being located in a separate cavity of         active material from each other electrode subgroup.

12C. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   the modulator is configured to be operated in a push-pull mode.

13C. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.         Group D: MZ with (optical devices of Group B);

1D. A Mach-Zehnder modulator comprising:

-   -   a left arm and a right arm:     -   the left arm comprising a left active optical waveguide, and;     -   the right arm comprising a right active optical waveguide;     -   wherein each of the left and right optical waveguides are         according to any para of Group B, and;     -   wherein the first doped semiconductor region of the left         waveguide is integral with the first doped semiconductor region         of the right waveguide in a common doped region located between         the left arm and the right arm.

2D. A Mach-Zehnder modulator according to para 1D, further comprising:

-   -   a common electrode attached to the common doped region.

3D. A Mach-Zehnder modulator according to para 1D, further comprising:

-   -   a plurality of common electrodes attached to the common doped         region along the waveguide.

4D. A Mach-Zehnder modulator according to para 3D, wherein:

-   -   the plurality of common electrodes are electrically connected to         each other.

5D. A Mach-Zehnder modulator according to para 2D, further comprising:

-   -   a single left electrode connected to the second doped         semiconductor region of the left waveguide, and;     -   a single right electrode connected to the second doped         semiconductor region of the right waveguide.

6D. A Mach-Zehnder modulator according to para 5D, wherein:

-   -   the left and right electrodes are each lumped electrodes or         traveling wave electrodes.

7D. A Mach-Zehnder modulator according to para 2D or para 3D, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second doped semiconductor region of the left         waveguide, and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second doped semiconductor region of the right         waveguide;     -   wherein the number of left lumped electrodes and the number of         right lumped electrodes are the same.

8D. A Mach-Zehnder modulator according to para 7D, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the plurality of central electrodes are         contained within a single cavity of active material.

9D. A Mach-Zehnder modulator according to para 3D, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second doped semiconductor region of the left         waveguide, and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second doped semiconductor region of the right         waveguide;     -   wherein the number of left lumped electrodes, the number of         right lumped electrodes, and the number of common electrodes are         the same.

10D. A Mach-Zehnder modulator according to para 9D, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the central electrode are contained         within a single cavity of active material.

11D. A Mach-Zehnder modulator according to para 9D, wherein:

-   -   the electrodes comprise a plurality of electrode subgroups, each         electrode subgroup comprising:     -   at least one of the plurality of right lumped electrodes;     -   at least one of the plurality of left lumped electrodes, and;     -   at least one of the plurality of central electrodes;     -   each electrode subgroup being located in a separate cavity of         active material from each other electrode subgroup.

12D. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   the modulator is configured to be operated in a push-pull mode.

13D. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.         Group E: MZ with doped Si walls

1E. A Mach-Zehnder modulator, comprising a left arm and a right arm:

-   -   the left arm comprising a left active optical waveguide, and;     -   the right arm comprising a right active optical waveguide;     -   each active optical waveguide comprising:     -   a SiGe central waveguide region;     -   a first wall of doped Si on a first side of the central region;     -   a second wall of doped Si on a second side of the central         region, and;     -   a first lateral region of doped Si adjacent to the first wall,     -   the first lateral region having the same doping as the first         wall, and;     -   a second lateral region of doped Si adjacent to the second wall;     -   the second lateral region having the same doping as the second         wall;     -   wherein, the first lateral region of the left optical waveguide         is integral with the first lateral region of the right optical         waveguide in a common doped region located between the arms of         the modulator.

2E. A Mach-Zehnder modulator according to para 1E, wherein:

-   -   the common doped region and the first walls are each n-doped         and;     -   the second walls are p-doped.

3E. A Mach-Zehnder modulator according to para 1E, wherein:

-   -   the common doped region and the first walls are each p-doped         and;     -   the second walls are n-doped.

4E. A Mach-Zehnder modulator according to any preceding para, further comprising:

-   -   a common electrode attached to the common doped region.

5E. A Mach-Zehnder modulator according to any one of paras 1E-3E, further comprising:

-   -   a plurality of common electrodes attached to the common doped         region along the waveguide.

6E. A Mach-Zehnder modulator according to para 5E, wherein:

-   -   the plurality of common electrodes are electrically connected to         each other.

7E. A Mach-Zehnder modulator according to any one of paras 1E-4E, further comprising:

-   -   a single left electrode connected to the second lateral region         of the left waveguide, and;     -   a single right electrode connected to the second lateral region         of the right waveguide.

8E. A Mach-Zehnder modulator according to para 5E, wherein:

-   -   the left and right electrodes are each lumped electrodes or         traveling wave electrodes.

9E. A Mach-Zehnder modulator according to para 4E, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second lateral region of the left waveguide,         and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second lateral region of the right waveguide;     -   wherein the number of left lumped electrodes and the number of         right lumped electrodes is the same.

10E. A Mach-Zehnder modulator according to para 9E, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the plurality of central electrodes are         contained within a single cavity of active material.

11E. A Mach-Zehnder modulator according to para 5E, further comprising:

-   -   a plurality of left lumped electrodes connected along the         waveguide to the second lateral region of the left waveguide,         and;     -   a plurality of right lumped electrodes connected along the         waveguide to the second lateral region of the right waveguide;     -   wherein the number of left lumped electrodes, the number of         right lumped electrodes, and the number of common electrodes are         the same.

12E. A Mach-Zehnder modulator according to para 9E, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the central electrode are contained         within a single of active material.

13E. A Mach-Zehnder modulator according to para 11E, wherein:

-   -   the electrodes comprises a plurality of electrode subgroups,         each electrode subgroup comprising:     -   at least one of the plurality of right lumped electrodes;     -   at least one of the plurality of left lumped electrodes, and;     -   at least one of the plurality of central electrode;     -   each electrode subgroup being located in a separate cavity of         active material from each other electrode subgroup.

14E. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   the modulator is configured to be operated in a push-pull mode.

15E. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.         Group F: MZ with isolated arms according to the optical         waveguide devices of Group A.

1F. A Mach-Zehnder modulator with two arms, comprising:

-   -   a left arm comprising a left active optical waveguide, and the         right arm comprising a right active optical waveguide;     -   each of the left and right optical waveguides are according to         any para of Group A;     -   wherein the left optical waveguide is electrically isolated from         the right optical waveguide by a central isolating region.

2F. A Mach-Zehnder modulator according to para 1F, wherein:

-   -   the first vertical regions are each n-doped and;     -   the second lateral regions are each p-doped.

3F. A Mach-Zehnder modulator according to para 1F, wherein:

-   -   the first vertical regions are each p-doped and;     -   the second lateral regions are each n-doped.

4F. A Mach-Zehnder modulator according to any preceding para, further comprising:

-   -   a first single left electrode connected to the first lateral         region of the left waveguide, and;     -   a first single right electrode connected to the first lateral         region of the right waveguide.     -   a second single left electrode connected to the second lateral         region of the left waveguide, and;     -   a second single right electrode connected to the second lateral         region of the right waveguide.

5F. A Mach-Zehnder modulator according to para 4F, wherein:

-   -   the first single left electrode, the first single right         electrode, the second single left electrode and the     -   second single right electrode are each a lumped electrode or a         traveling wave electrode.

6F. A Mach-Zehnder modulator according to any one of paras 1F-3F, further comprising:

-   -   a first plurality of left lumped electrodes connected along the         first lateral region of the left waveguide, and;     -   a first plurality of right lumped electrodes connected along the         first lateral region of the right waveguide;     -   a second plurality of left lumped electrodes connected along the         second lateral region of the left waveguide, and;     -   a second plurality of right lumped electrodes connected along         the second lateral region of the right waveguide;     -   wherein the number of first left lumped electrodes, the number         of first right lumped electrodes, the number of second left         lumped electrodes, and the number of second right lumped         electrodes are the same.

7F. A Mach-Zehnder modulator according to para 6F, wherein:

-   -   wherein the plurality of first left lumped electrodes, the         plurality of first right lumped electrodes,     -   the plurality of second left lumped electrodes, and the         plurality of second right lumped electrodes are located in a         single cavity of active material.

8F. A Mach-Zehnder modulator according to para 6F, wherein:

-   -   the electrodes comprises a plurality of electrode subgroups,         each electrode subgroup comprising:     -   at least one of the plurality of first right lumped electrodes;     -   at least one of the plurality of first left lumped electrodes;     -   at least one of the plurality of second right lumped electrodes;     -   at least one of the plurality of second left lumped electrodes,         and; each electrode subgroup being located in a separate cavity         of active material from each other electrode subgroup.

9F. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.         Group G: MZ with isolated arms according to the optical devices         of Group B

1G. A Mach-Zehnder modulator comprising:

-   -   a left arm and a right arm:     -   the left arm comprising a left active optical waveguide, and;     -   the right arm comprising a right active optical waveguide;     -   wherein each of the left and right optical waveguides are         according to any para of Group B, and;     -   wherein the left waveguide is electrically isolated from the         right waveguide by a central isolating region.

2G. A Mach-Zehnder modulator according to para 1G, wherein:

-   -   the first semiconductor regions are each n-doped and;     -   the second semiconductor regions are each p-doped.

3G. A Mach-Zehnder modulator according to para 1G, wherein:

-   -   the first semiconductor regions are each p-doped and;     -   the second semiconductor regions are each n-doped.

4G. A Mach-Zehnder modulator according to any one of paras 1G-3G, further comprising:

-   -   a first single left electrode connected to the first         semiconductor region of the left waveguide, and;     -   a first single right electrode connected to the first         semiconductor region of the right waveguide.     -   a second single left electrode connected to the second         semiconductor region of the left waveguide, and;     -   a second single right electrode connected to the second         semiconductor region of the right waveguide.

5G. A Mach-Zehnder modulator according to para 4G, wherein:

-   -   the first single left electrode, the first single right         electrode, the second single left electrode and the second         single right electrode are each a lumped electrode or a         traveling wave electrode.

6G. A Mach-Zehnder modulator according to any one of paras 1G-3G, further comprising:

-   -   a first plurality of left lumped electrodes connected along the         first semiconductor region of the left waveguide, and;     -   a first plurality of right lumped electrodes connected along the         first semiconductor region of the right waveguide;     -   a second plurality of left lumped electrodes connected along the         second semiconductor region of the left waveguide, and;     -   a second plurality of right lumped electrodes connected along         the second semiconductor region of the right waveguide;     -   wherein the number of first left lumped electrodes, the number         of first right lumped electrodes, the number of second left         lumped electrodes, and the number of second right lumped         electrodes are the same.

7G. A Mach-Zehnder modulator according to para 6G, wherein:

-   -   the plurality of first left lumped electrodes, the plurality of         first right lumped electrodes, the     -   plurality of second left lumped electrodes, and the plurality of         second right lumped electrodes are contained with a single         cavity of active material.

8G. A Mach-Zehnder modulator according to para 6G, wherein:

-   -   the electrodes comprises a plurality of electrode subgroups,         each electrode subgroup comprising:     -   at least one of the plurality of first right lumped electrodes;     -   at least one of the plurality of first left lumped electrodes,         and;     -   at least one of the plurality of second right lumped electrodes;

at least one of the plurality of second left lumped electrodes, and;

-   -   each electrode subgroup being located in a separate cavity of         active material from each other electrode subgroup.

9G. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.

Group H: MZ with isolated arms, the arms including doped Si walls

1H. A Mach-Zehnder modulator, comprising a left and a right arm, the left and right arms being parallel, wherein:

-   -   each of the two arms comprises at least one active optical         waveguide, each active optical waveguide comprising:     -   a SiGe central waveguide region;     -   a first wall of doped Si on a first side of the central region;     -   a second wall of doped Si on a second side of the central         region, and;     -   a first lateral region of doped Si adjacent to the first wall,     -   the first lateral region having the same doping as the first         wall, and;     -   a second lateral region of doped Si adjacent to the second wall;     -   the second lateral region having the same doping as the second         wall;     -   wherein the doping in the first wall is of an opposite sense to         the doping in second wall, and;     -   wherein, the left optical waveguide is electrically isolated         from the right optical waveguide by a central isolating region.

2H. A Mach-Zehnder modulator according to para 1H, wherein:

-   -   the first walls are each n-doped and;     -   the second walls are each p-doped.

3H. A Mach-Zehnder modulator according to para 1H, wherein:

-   -   the first walls are each p-doped and;     -   the second walls are each n-doped.

4H. A Mach-Zehnder modulator according to any one of paras 1H-3H, further comprising:

-   -   a first single left electrode connected to the first lateral         region of the left waveguide, and;     -   a first single right electrode connected to the first lateral         region of the right waveguide.     -   a second single left electrode connected to the second lateral         region of the left waveguide, and;     -   a second single right electrode connected to the second lateral         region of the right waveguide.

5H. A Mach-Zehnder modulator according to para 4H, wherein:

-   -   the first and second left and right electrodes are each lumped         electrodes or traveling wave electrodes

6H. A Mach-Zehnder modulator according to para 4H, further comprising:

-   -   a first plurality of left lumped electrodes connected along the         first lateral region of the left waveguide, and;     -   a first plurality of right lumped electrodes connected along the         first lateral region of the right waveguide;     -   a second plurality of left lumped electrodes connected along the         second lateral region of the left waveguide, and;     -   a second plurality of right lumped electrodes connected along         the second lateral region of the right waveguide;     -   wherein the number of first left lumped electrodes, the number         of first right lumped electrodes, the number of second left         lumped electrodes, and the number of second right lumped         electrodes are the same.

7H. A Mach-Zehnder modulator according to para 9H, wherein:

-   -   the plurality of left lumped electrodes, the plurality of right         lumped electrodes, and the plurality of central electrodes are         contained within a single cavity of active material.

8H. A Mach-Zehnder modulator according to para 9H, wherein:

-   -   the electrodes comprises a plurality of electrode subgroups,         each electrode subgroup comprising:     -   at least one of the plurality of first right lumped electrode;     -   at least one of the plurality of first left lumped electrode,         and;     -   at least one of the plurality of second right lumped electrodes;     -   at least one of the plurality of second left lumped electrodes,         and;     -   each electrode subgroup being located in a separate cavity of         active material from each other electrode subgroup.

9H. A Mach-Zehnder modulator according to any preceding para, wherein:

-   -   each arm of the modulator is configured to be operated as a         phase modulator.

Although exemplary embodiments of a Mach-Zehnder waveguide modulator have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a Mach-Zehnder modulator constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof. 

What is claimed is:
 1. A Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a PIN junction region and a plurality of electrodes for providing a bias across the PIN junction to enable control of the phase of light travelling through the respective PIN junction region via dispersion; wherein each of the PIN junctions is formed from: a first semiconductor region corresponding to either a p-doped region or an n-doped region; a second semiconductor region corresponding to the other of the p-doped or n-doped region; and a central SiGe waveguide region; wherein the first semiconductor region of the left optical waveguide is integral with the first semiconductor region of the right optical waveguide in a region between the left and right arms, forming a common doped region; and the plurality of electrodes is configured to apply a forward bias across one or both of the PIN junctions.
 2. A Mach-Zehnder waveguide modulator according to claim 1, wherein: at least one of the first semiconductor region or second semiconductor region includes a vertical doped portion which extends along the side of the waveguide.
 3. A Mach-Zehnder waveguide modulator according to claim 2, wherein the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped portion.
 4. A Mach-Zehnder waveguide modulator according to claim 2, wherein the vertical doped portion extends in the vertical direction along the entire side of the waveguide.
 5. A Mach-Zehnder waveguide modulator according to claim 2, wherein, for a given waveguide arm of the modulator, no more than one side of the waveguide comprises a vertical doped portion.
 6. A Mach-Zehnder waveguide modulator according to claim 1, wherein the Mach-Zehnder waveguide modulator further comprises: an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region; wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region.
 7. A Mach-Zehnder waveguide modulator according to claim 1, wherein the Mach-Zehnder waveguide modulator further comprises: an intervening lightly n-doped semiconductor region located between the n-doped semiconductor region and the central waveguide region; wherein the intervening lightly n-doped semiconductor region has a lower dopant concentration than the n-doped semiconductor region.
 8. A Mach-Zehnder waveguide modulator according to claim 7, wherein: the Mach-Zehnder waveguide modulator further comprises an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region, wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region; and the central waveguide region, the n-doped semiconductor region, the p-doped semiconductor region, the lightly p-doped semiconductor region, and the lightly n-doped semiconductor region are all composed of SiGe.
 9. A Mach-Zehnder waveguide modulator according to claim 1, wherein: the first semiconductor region; and the second semiconductor region are each composed of Si.
 10. A Mach-Zehnder waveguide modulator according to claim 1, wherein: the central SiGe waveguide region, the first semiconductor region, and the second semiconductor region are all composed of SiGe.
 11. A Mach-Zehnder waveguide modulator according to claim 1, wherein the plurality of electrodes includes a common electrode located at the common doped region.
 12. A Mach-Zehnder waveguide modulator according to claim 1 wherein, for each waveguide arm: the first semiconductor region includes a first lateral portion extending laterally away from the waveguide wall on a first side of the waveguide; the second semiconductor region includes a second lateral portion extending laterally away from the waveguide wall on a second side of the waveguide; and the plurality of electrodes comprises: a first electrode located directly on top of the first lateral portion; and a second electrode located directly on top of the first lateral portion.
 13. A Mach-Zehnder waveguide modulator according to claim 1, wherein the plurality of electrodes includes a plurality of lumped electrodes.
 14. A Mach-Zehnder waveguide modulator according to claim 1, configured to operate in a push-pull mode.
 15. A Mach-Zehnder waveguide modulator comprising: a left arm including a left SiGe optical waveguide, and a right arm including a right SiGe optical waveguide; wherein each of the left and right optical waveguides comprises a PIN junction region and a plurality of electrodes for providing a bias across the PIN junction to enable control of the phase of light travelling through the respective PIN junction region via dispersion; wherein each of the PIN junctions is formed from: a first semiconductor region corresponding to either a p-doped region or an n-doped region; a second semiconductor region corresponding to the other of the p-doped or n-doped region; and a central SiGe waveguide region; wherein the first semiconductor region of the left optical waveguide is electrically isolated from the first semiconductor region of the right optical waveguide by a central isolated region between the left and right arms; and the plurality of electrodes is configured to apply a forward bias across one or both of the PIN junctions.
 16. A Mach-Zehnder waveguide modulator according to claim 15, wherein: at least one of the first semiconductor region or second semiconductor region includes a vertical doped portion which extends along the side of the waveguide.
 17. A Mach-Zehnder waveguide modulator according to claim 16, wherein the vertical doped portion only extends in the vertical direction along a portion of the side wall of the waveguide, such that the central SiGe waveguide region has a greater height than the vertical doped portion.
 18. A Mach-Zehnder waveguide modulator according to claim 16, wherein the vertical doped portion extends in the vertical direction along the entire side of the waveguide.
 19. A Mach-Zehnder waveguide modulator according to claim 15, wherein the Mach-Zehnder waveguide modulator further comprises: an intervening lightly p-doped semiconductor region located between the p-doped semiconductor region and the central waveguide region; wherein the intervening lightly p-doped semiconductor region has a lower dopant concentration than the p-doped semiconductor region.
 20. A Mach-Zehnder waveguide modulator according to claim 15, wherein the Mach-Zehnder waveguide modulator further comprises: an intervening lightly n-doped semiconductor region located between the n-doped semiconductor region and the central waveguide region; wherein the intervening lightly n-doped semiconductor region has a lower dopant concentration than the n-doped semiconductor region. 